What you buy and how you apply could make all the difference in whether you develop skin cancer. Here, everything you must know about sun protection.

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If you don't already take steps to protect yourself from skin cancer every single day, now's the time to start. "We're seeing an epidemic of melanoma – the deadliest form of skin cancer-- in young women," says Lynn Schuchter, M.D., an oncologist at the Abramson Cancer Center at the University of Pennsylvania. And the incidence of non-melanoma skin cancer has more than doubled in women under age 40 in less than 30 years, according to recent research from the Mayo Clinic in Rochester, MN. "There's been an increase in use of tanning salons," explains Schuchter. "Plus, people are more scantily clad these days" -- which leaves us more exposed to the sun's harmful rays during everyday activities such as walking the dog or waiting for the kids at the school bus stop. Fortunately, what scientists know about protecting skin from the sun's cancer-causing ultraviolet (UV) rays is also increasing, and that knowledge has gone into the making of this season's hottest new sunscreens. But if you think simply slathering on protection will keep you safe, think again. Check out the following surprising facts about sunscreen and your skin. They might just save your life. Yes, it's truly that serious.

Your sunscreen may not protect against UVA rays.
All sunscreens protect against UVB rays, the type of UV light that causes sunburns and skin cancer. But not all of them protect against UVA rays, which penetrate skin more deeply and cause skin cancer and photodamage such as fine lines, wrinkles, roughness, and brown patches. "We used to think that UVA rays only made us tan, so we ignored them," explains Kerry M. Hanson, Ph.D., senior research scientist in chemistry at the University of California at Riverside. "Now we know they can also cause skin cancer and are a primary culprit of premature aging."The good news: More and more sunscreens are being formulated to block both UVA and UVB rays; those that do typically carry the label "broad-spectrum protection." New ingredients are making this step toward total UV coverage possible.For instance, last summer, the FDA finally approved a product with Mexoryl SX, a UVA-blocking ingredient long used in Europe and Canada; it can now be found in L'Oréal's Anthelios SX. Plus, finding a full-coverage sunscreen will be easier this summer since the American Academy of Dermatology has added its "Seal of Recognition" to brands that provide broad-spectrum protection, are water resistant, and have a Sun Protection Factor (SPF) of at least 15 -- which allows you to remain in the sun about 15 times longer without burning than if you weren't wearing sunscreen (keep in mind that lighter skin tones burn faster).

Not reapplying sunscreen can increase your risk of skin cancer.
Here's how it happens: Some sunscreens get fully absorbed into skin after about two hours, at which point they're no longer effective at shielding skin against the sun's rays. "When UV rays hit that penetrated layer of sunscreen, they react with the sunscreen's molecules, which activates the formation of free radicals, cell-damaging atoms that can cause aging, wrinkling, even skin cancer," explains Hanson. "The perfect sunscreen would act like latex paint—it would stay on the surface of the skin and then peel off."Since no one's invented that formula yet, the simple solution if you're spending time outside is to diligently reapply sunscreen every two hours. As long as the product sits on the skin's surface, it does a good job of blocking harmful UV rays. Also, use a formula that contains antioxidants such as vitamins C and E; these compounds have been shown to help inhibit the formation of free radicals.

Having a family history of breast or ovarian cancer ups your risk of melanoma.
The same gene mutation that increases your risk of hereditary breast and ovarian cancer—BRCA2—is also associated with a two-and-a-half times greater risk of melanoma, the number one cancer among 20- to 30-year-olds. "More and more, we're understanding that cancer is a genetic disorder, and that many of these cancers are linked," says Schuchter. While she doesn't recommend that everyone rush to be screened for BRCA2, if a close relative (such as a parent, grandparent, or sibling) has had breast or ovarian cancer or melanoma, Schuchter does advise taking added precautions, such as seeing a dermatologist every six to 12 months to check for skin malignancies and avoiding direct sun exposure whenever possible.

You're not fully safe in your car, home, or office.
While standard windows block UVB rays, a lot of UVA light wafts into buildings and cars. "Potentially, the incidental sun exposure we receive during everyday indoor activities can lead to premature aging and skin cancer," says Henry Lim, M.D., chairman of the dermatology department at Henry Ford Hospital in Detroit. In fact, men who spend a lot of time behind the wheel have a higher incidence of skin cancer on the left side of their body, probably because of sunlight coming through the driver's-side car window, according to a new study from the Saint Louis University School of Medicine. Lim's advice: "Don't sit by a window for more than an hour a day, whether it's a sunny day or not -- UVA rays penetrate glass regardless of the weather." If you must sit in direct sunlight while indoors, close the blinds during peak sun hours (10 a.m. to 4 p.m.), and wear sunscreen with SPF 15 or higher. If you have a long commute, consider updating clear glass car windows with tinted glass, which reduces UV transmission by about 25 percent.

Food can protect your skin.
Consuming plenty of tomatoes, dark chocolate, cocoa, green tea, and pomegranate extract could give the body an added defense against skin cancer. "These foods contain flavonoids and carotenoids, compounds that act as antioxidants and UV filters," says Wilhelm Stahl, Ph.D., of Heinrich-Heine University in Düsseldorf, Germany. In one of Stahl's studies, a group of women who added a flavonol-rich cocoa to their daily diet for three months experienced 25 percent less skin reddening when exposed to UV light; those who drank a flavonol-poor cocoa saw no change in their skin's reaction to UV rays. This "internal sunscreen" will give you a natural SPF of about 2 or 3, Stahl says. "That may seem low, but its cumulative effect is getting more attention."

You can run away from skin cancer.
One more reason to hop on the treadmill: Studies have linked exercise with a lower risk of developing cancer, including melanoma. Most recently, researchers at Rutgers University found that mice who took regular jogs on a running wheel and were then exposed to UV light developed 30 percent fewer tumors -- and smaller, slower-growing tumors – than mice without a running wheel in their cage. "Exercise enhances the biochemical mechanisms that trigger sun-damaged cells to self-destruct before they can become cancerous," explains study author Allan Conney, Ph.D.

Exercise's fat-burning powers might also play a role. "The active mice had less fat tissue, and there may be a relationship between fat and cancer formation," Conney says. Though experts don't yet know how much physical activity is enough to get the skin-protective benefits, Conney recommends getting exercise at least three times per week.

Your sweet tooth can make you more vulnerable.
If you have high blood sugar -- even if you're not diabetic or obese—you could be at greater risk of developing melanoma, among other cancers. In a recent 17-year study, Swedish researchers found that women with the highest glucose or blood sugar levels had a 26 percent greater risk of developing all types of cancer than women with the lowest blood sugar. "Excess glucose triggers the production of the hormone insulin, which stimulates the growth of cells -- including cancerous ones," explains lead researcher Pär Stattin, M.D., of Sweden's Umeå University Hospital. A quick blood test can determine whether your blood sugar levels are high. If so, simple lifestyle changes such as cutting back on sweets and exercising regularly can help bring levels back into a healthy range. If not, consider adopting these healthy habits anyway: They have cancer-fighting powers no matter your individual risk factors.

Whether you're a fair-skinned redhead or a brunette who bronzes easily, the American Academy of Dermatology recommends that everyone follow these safe-sun strategies.

Fifteen to 30 minutes before heading outdoors, apply a generous amount (about a shot-glass full) of sunscreen to all exposed skin—including face, ears, and hands. Choose a sunscreen that has an SPF of at least 15, provides broad-spectrum coverage (UVB and UVA protection), and is water resistant, even if you're not swimming; research shows that perspiration makes your skin more photosensitive—and therefore more likely to burn.

Reapply every two hours, even on cloudy days (80 percent of the sun's UV rays pass through clouds) and especially after swimming and sweating—even water-resistant formulas won't protect you after about 80 minutes.

Wear protective clothing—such as a long-sleeved shirt, pants, and a wide-brimmed hat—and sunglasses when possible.

Seek shade when the sun's rays are strongest, between 10 a.m. and 4 p.m.

Avoid tanning beds, which have been linked to skin cancer and wrinkling.

Check your birthday suit on your birthday. If you notice any changing, growing, or bleeding of moles on your skin, see a dermatologist ASAP (since all are signs of melanoma). And schedule a mole check with your dermatologist, who can examine hard-to-see places such as your back or behind your ears.

"The first and last time I forgot to apply sunscreen to my eyelids was on a trip to Hawaii after high school. My friend and I spent an entire afternoon lying out and playing in the water. The next morning our eyes were swollen shut! Neither of us could hold our eyelids open so we spent the morning in the hotel icing our eyes with cans of chilled Hawaiian Sun."
—Coline LeConte, 31, Los Gatos, CALesson Learned: Remember your eyelids... and your ears and your lips.

"During a vacation in Puerto Rico, I fell asleep for the afternoon on a lounge chair, facedown. I'd applied sunscreen, but somehow I missed that little crease behind my knees. They got so burned, I couldn't walk, sit, go to the bathroom, etc., without shrieking in agony for three days."
—Audrey Slater, 39, REDBOOK fashion directorLesson Learned: Always use too much sunscreen—that way you can't help but get it everywhere.

"For spring break my senior year of college, I drove a carful of my friends down to Florida from Virginia. As the weather warmed up, I rolled down my window. It never occurred to me to put on sunscreen. By the time we got to Key West, my left ear and the left side of my face—and only the left side—was beet red.What's worse, a few days later it started peeling—gross!"
—Carin Gorrell, 31, REDBOOK health editorLesson Learned: Hit the sunscreen before you hit the road.

"I tried out one of those totally easy spray-on sunscreens during my vacation this spring. It worked great -- except for one little application glitch. I must have held the nozzle too close to my chest, because the next day I had two distinct circles of lighter flesh in my cleavage. It looked (and still looks) ridiculous!"
—Stacy Morrison, 38, REDBOOK editor-in-chiefLesson Learned: Hold a spray-on sunscreen bottle about 6 to 8 inches away from skin and apply liberally to ensure complete coverage.